A photoresist mask is used in semiconductor device fabrication to protect portions of an underlying material from an etching compound during an etching process. A light-sensitive photoresist material is formed on the surface of the semiconductor structure and portions of the photoresist material are exposed to light through a photolithographic mask. The light that is used to expose the photoresist material through the photolithographic mask typically has a specific wavelength, such as light in the ultraviolet spectrum. The portions of the photoresist which are exposed to the light undergo a photochemical reaction which alters the physical characteristics of the exposed portions. For positive photoresist, the chemical characteristics of photoresist material that is exposed to the light is changed, making the photoresist more easily dissolvable in a developer solution. Those portions of the photoresist material that are not exposed to the light will become more difficult to dissolve in the developer solution. The photoresist material is then developed and the exposed portions of the photoresist material are washed away to form a photoresist mask. Generally an organic anti-reflection layer is also applied during the photoresist process to prevent reflection of light from the underlying material.
The photoresist mask forms a pattern over the underlying material on the wafer and serves to protect portions of the underlying material during an etching process. Portions of the underlying material not protected by the photoresist mask are then etched away or treated in some other manner during the semiconductor device fabrication process. Typically, etching is done with a chemical agent or plasma. In plasma etching, the semiconductor structure is placed in an atmosphere containing a chemical plasma and the wafer is bombarded with ions to remove the unprotected portions of the underlying material. After the underlying material is etched, the photoresist mask is removed from the wafer leaving the desired patterned features in the underlying material on the wafer.
One use of a photoresist mask is to form metal structures as part of the semiconductor device being fabricated on a silicon substrate. A layer of metal is deposited on the silicon substrate and a photoresist mask having the desired metal structure pattern is formed on the metal layer. A metal etching process is used to remove the unprotected metal portions surrounding the desired metal structures. The photoresist mask is then removed. The remaining metal forms the metal structures in the desired pattern. However, the photoresist mask is partially etched away inwardly during the metal etching process which reduces the thickness of the protective photoresist mask over the underlying metal structures. If the thickness of photoresist mask is not sufficient, the mask may be totally etched away, which will result in etching of the underlying metal structures in undesired locations.
The geometries of the desired metal structures also decrease as the minimum photolithographic geometries decrease. In conventional photolithography, a thinner layer of resist material is used in the photoresist mask for the smaller geometries. The proportion of the photoresist mask that is etched away during the etching process is greater with the smaller geometries. As a result, the metal structures formed with the etching process are narrower in width than desired and may not be uniform or well defined because of insufficient thickness in the photoresist mask which is intended to protect the patterned metal structures.
The metal structures are also reduced in width by inadvertent etching in a lateral direction inward from the sidewalls of the metal structures beneath the mask. Although the vertical etch rate through the metal is greater than the lateral or horizontal etch rate of the metal, the sidewalls of the metal structures are etched to some degree. Typically, the metal structures will have jagged or generally poor quality edges resulting from metal that has been etched away from the sidewalls of the metal structures. Under certain conditions, metallic residue can also be formed in open fields of the semiconductor structure when a relatively large area of metal has been etched away.
To reduce the amount of lateral etching and metallic residue, carbon is typically introduced into the chemical plasma during the metal etching process. The carbon bonds to the sidewalls of the metal structures during the metal etching process and passivates the sidewalls of the metal structures. The carbon passivation prevents lateral etching of the metal layer during the metal etching process. The carbon passivation also prevents the accumulation of metallic residue in other locations. However, the introduction of carbon into the chemical plasma may complicate the etching process because carbon must typically be introduced into a gaseous mixture forming the chemical plasma. The carbon introduced into the chemical plasma alters the etching chemistry of the chemical plasma, and the alteration could require changes in or variations to the fabrication process.
A hard mask may be used to protect the metal structures when a thin layer of photoresist material is used in semiconductor wafer fabrication. A hard mask is similar to a conventional photoresist mask but the hard mask is more resistant to etching during the metal etching process. Therefore, the hard mask may be used to produce more uniform and defined metal structures than are possible with a conventional photoresist mask. Typically, the hard mask is a dielectric material such as silicon dioxide or silicon nitride, or another insulating compound formed from silicon, oxygen, nitrogen, tungsten and other metals. However, it is not believed that carbon has been included in the hard mask material.
The hard mask is made by first forming a layer of the dielectric material on the metal layer which has been deposited on the wafer. A photoresist mask is then formed on the dielectric material. The dielectric material is etched from the photoresist mask to form the hard mask having the same pattern as the photoresist mask. The hard mask is then used in the metal etching process to protect the desired underlying metal structure being fabricated while etching away the surrounding metal layer to form the desired metal structure. The metal etching process etches the hard mask at a slower rate than the metal etching process typically etches the photoresist mask. However, the hard mask is still etched to some degree along the sidewalls and on other exposed portions of the hard mask. The desired metal structure being fabricated is better protected from the metal etching process by using the hard mask rather than a conventional photoresist mask. As a result, the metal structure is more uniform and well defined than a metal structure formed with only a photoresist mask, and the size of the metal structure is closer to the original size of the photoresist mask above the metal structure.
It is with respect to these and other considerations that have given rise to the present invention.